Organic sulphides are widely used in a very large number of fields in the chemical industry and in particular the petrochemical industry. In particular, dimethyl disulphide is used as an agent for sulphiding catalysts for the hydrotreatment of petroleum feedstocks, as a feedstock additive for steam cracking, to cite just a few of the possible uses of this compound.
Compared with other products used in these applications, for instance commercial di-tert-alkyl polysulphides, organic sulphides, and in particular DMDS, have many advantages, in particular a high sulphur content (68%) and non-coking degradation products (CH4, H2S in the case of DMDS). Furthermore, in these applications, DMDS results in performance levels that are generally superior to other commercial products normally used, for example di-tert-alkyl polysulphides.
Among the known methods for the synthesis of organic sulphides, a particularly efficient and economical method is the oxidation of alkyl mercaptans with sulphur, for example for the synthesis of DMDS, the oxidation of methyl mercaptan with sulphur according to the following reaction:

This oxidation of alkyl mercaptans with sulphur, catalysed by organic or inorganic, homogeneous or heterogeneous, basic agents under batchwise or continuous conditions, is accompanied by a release of hydrogen sulphide and also of dialkyl polysulphides (for example dimethyl polysulphides CH3SxCH3 in the case of the synthesis of DMDS) with a sulphur rank x of greater than 2.
In order to manufacture DMDS according to this process of oxidation with sulphur with high yields and a limited production of DMPS (dimethyl polysulphides with a rank greater than 2), patent EP 0 446 109 describes a preparation process comprising two reaction regions interrupted by an intermediate degassing region and followed by a distillation region, in order to eliminate the unwanted by-products.
Although giving a good performance level in terms of yield and selectivity for DMDS, it turns out that this process results in a final product comprising a not insignificant amount of methyl hydrodisulphide (CH3SSH). This amount generally varies between approximately 200 ppm and approximately 800 ppm, in particular when high DMDS productivities are sought.
The result of this impurity, which can be considered to be a reaction intermediate, is a slow decomposition over time to give methyl mercaptan and dimethyl trisulphide, by reaction with dimethyl disulphide, according to the following reaction:CH3SSH+CH3SSCH3→CH3SH+CH3SSSCH3 
The instability of methyl hydrodisulphide (CH3SSH, CAS No.: 6251-26-9) is, moreover, mentioned in the literature by H. Bohme and G. Zinner, in “Justus Liebigs Annalen der Chemie”, 585, (1954), 142-9, in which the methyl hydrodisulphide CH3SSH decomposes at ambient temperature, by reacting with itself, to give dimethyl trisulphide and H2S according to the reaction:2CH3SSH→H2S+CH3SSSCH3 
In the case of the synthesis of DMDS by oxidation with sulphur, the very low concentrations of CH3SSH do not enable this product to react with itself, and the reaction with the product predominantly present in the medium, DMDS, is highly favoured.
The increase in the methyl mercaptan content in the DMDS makes the synthesis of DMDS described in application EP 0 976 726 not very cost effective, owing to the volatile impurities in the DMDS (traces of methyl mercaptan and traces of dimethyl sulphide (DMS)) which are eliminated in an additional distillation step.
Indeed, its impurities give DMDS a very unpleasant and aggressive odour, which is regarded as a significant cause of trouble during the handling of this product by users.
However, the DMDS obtained, freed of the volatile impurities, still contains traces of CH3SSH. In addition, the simple elimination of the impurities by distillation results in considerable DMDS yield losses. This is because CH3SSH is much less volatile than methyl mercaptan and than DMS, and has a boiling point of 108° C. very close to that of DMDS (boiling point between 107° C. and 110° C. at ambient pressure). These products (DMDS and CH3SSH) are therefore very difficult to separate.
The main objective of the present invention is therefore to provide a process which makes it possible to reduce, or even to eliminate to a very large extent as far as completely, the alkyl hydrodisulphide impurity, for example CH3SSH in the case of the synthesis of DMDS.
Another objective of the present invention is a process for eliminating the alkyl hydrodisulphide impurity, without a significant loss of dialkyl disulphide yield.